10.1 Chemical Equations

Transcription

1 Section 10.1 Equations 10.1 Equations Developing the language of chemistry reactions and change Think of the element symbols as the alphabet of chemistry and the formulas as the words of chemistry. s are the sentences of chemistry; they tell the story of the material world. All the complex changes we see in the material world are the result of reactions. The big story of the world is the interrelationships among all the countless reactions that occur. To understand that story, even in part, we need the language of s, which really are the sentences of chemistry. A reaction is any process of change. reactions convert one or more substances into new substances. Atoms are not created or destroyed in the process. They are simply rearranged to form new substances. In general, we represent a reaction by a as follows: s and reactions The reactants are the starting materials used in the reaction. The products are the substances that are produced by the reaction. The reactants are always written on the left side of the arrow and the products are written on the right side. The arrow indicates the direction of the reaction, pointing from the reactants to the products. It means that the reactants give, yield, or react to form the products. reaction: the process that creates changes. : an expression that describes the changes that happen in a reaction. reactants: the starting materials or substances in a reaction, appearing on the left side of the. products: the materials or substances resulting from a reaction, appearing on the right side of the. 296 A NATURAL APPROACH TO CHEMISTRY

2 Constructing a An example reaction Let s construct the for one of the reactions that takes place in a fire. In this reaction, oxygen (O 2 ) from the atmosphere reacts with carbon (C) from wood to produce carbon dioxide gas (CO 2 ). In this reaction, O 2 and C are the reactants since they are consumed by the reaction. Carbon dioxide (CO 2 ) is the product. In words, the reaction can be written as follows: Carbon reacts with oxygen to produce carbon dioxide. We can write the expression in a more compact way using an arrow: with formulas carbon + oxygen carbon dioxide However, the use of the formulas for each substance involved in the reaction is the most accurate and least ambiguous way to describe the reaction. For our reaction, we write the general relationship with the using formulas: C + O 2 CO 2 Interpreting a The diagram below summarizes the interpretation of the. Why we use formulas Energy may appear on either side of a Using formulas to write the gives us a very compact way to express reactions. It also gives us a clear picture of the types of atoms that are involved in the reaction and how they are bonded together in both the products and the reactants. Does the C + O 2 CO 2 give a complete representation of burning wood? When we look at wood burning in a fire, we can see that fire gives off light, and we can feel that fires gives off heat. So, if the C + O 2 CO 2 does represent the phenomenon of wood burning, then where is the heat and where is the light? Later in the chapter we will add energy to the and explicitly recognize the energy as either a reactant (absorbed) or a product (given off). A NATURAL APPROACH TO CHEMISTRY 297

3 Section 10.1 Equations Conservation of mass reactions conserve mass The number of atoms in a reaction does not change One of the fundamental properties of chemistry is conservation of mass. In fact, this property is so important that scientists call it the law of conservation of mass. The law states that the total mass of the reactants is equal to the total mass of the products. In other words, mass is neither created nor destroyed during a reaction. The fundamental building block of chemistry and reactions is the atom. Conservation of mass really means that a reaction does not change the total number or type of atoms. A reaction rearranges the atoms in the reactants into a new arrangement of the same atoms in the products. Burning hydrogen in oxygen Beginning a For example, consider hydrogen burning in oxygen. This is a reaction that yields water as the product. It is also the reaction that fuels the space shuttle. The white smoke that comes out of the shuttle engines is not actually smoke, but water vapor, which is the product of the reaction. To start developing a we write down the reaction between hydrogen and oxygen with the correct formulas for participating substances. An unbalanced This correctly shows that water can be created by combining hydrogen and oxygen, but it does not satisfy the law of conservation of mass. There are two oxygen atoms in the reactants and only one in the products! law of conservation of mass: law that states that the total mass of reactants (starting materials) and the total mass of products (materials produced by the reaction) is the same. 298 A NATURAL APPROACH TO CHEMISTRY

4 Reading s Why an unbalanced is inaccurate Balanced means the same number of atoms appear on both sides Unbalanced s One problem with the H 2 + O 2 H 2 O is that if you mixed one mole of oxygen and one mole of hydrogen, you would not get one mole of water molecules. You would actually get one mole of water molecules and a half a mole of leftover oxygen molecules. You get leftover oxygen molecules because there are more oxygen atoms in the reactants than there are in the products. The chart below summarizes the reaction by counting the numbers of each atom on both sides of the reaction. The number of hydrogen atoms is the same in both the reactants and the products. We therefore can say that hydrogen atoms are balanced. The H 2 + O 2 H 2 O is an unbalanced. The unbalanced tells you what substances are involved in the reaction, but it does not tell you how much of each are involved. To make it balanced, we need to adjust the number of hydrogen molecules, oxygen molecules, and water molecules until there are the same number of each type of atom on both sides of the. The number of molecules in a is given by the coefficient in front of the formula. For example, the following might appear in a reaction to describe two hydrogen molecules, one oxygen molecule, or three water molecules: versus subscripts The subscript tells you how many atoms are in the molecule. For example the subscript 2 in H 2 tells you there are two hydrogen atoms in a hydrogen molecule. The coefficient tells you how many molecules there are in the reaction. If 2H 2 appears in a, then you know there are two molecules of hydrogen for a total of four atoms of hydrogen (two molecules two atoms per molecule). unbalanced : a that does not satisfy the law of conservation of mass in which the number of each type of atom on the reactant side of the does not equal the same number for each atom on the product side. A NATURAL APPROACH TO CHEMISTRY 299

5 Section 10.1 Equations A balanced The balanced By multiplying the hydrogen molecule by a factor of 2 and the water molecule by a factor of 2, we obtain a that satisfies mass conservation. Balanced s conserve mass The balanced correctly tells you amounts This is balanced. There are two rules for balancing s: 1. Do not change the subscripts in the formulas. This would change the substances involved in the reaction, in essence making a different reaction. For example, if we change H 2 O to H 2 O 2, we change water to hydrogen peroxide. 2. Do change the coefficients to adjust the number of each molecule in the reaction. This balances the reaction without changing the substances involved. The balanced tells us that when we combine two molecules of hydrogen with one molecule of oxygen, we can obtain two molecules of water. This is the complete recipe for making water. It correctly represents the amount of each substance in the reaction, and it satisfies mass conservation because the same number of each type of atom appears on both the reactant and product sides of the. balanced : a that satisfies the law of conservation of mass, when the total numbers of atoms of each element are the same on both the reactant and product sides of a. 300 A NATURAL APPROACH TO CHEMISTRY

6 Balancing s Why you need to balance s of one apply to whole molecules Balance pure elements last Balance one element at a time The goal of balancing a is to get the number of each type of atom to be the same on both the reactant and product sides of the. This is really just counting atoms on either side, then adjusting the coefficients until the numbers match. At this point, the process of balancing a is trial and error. You must guess and try different coefficients until it works out. For many reactions this approach works fine. However, for reactions that involve a number of reactants and products, a more structured approach is needed. We will develop it in the next few pages. When a single molecule is in a, the coefficient of 1 is not written. For example, O 2 in a means one molecule of oxygen (which contains two atoms). The coefficient of 2 in front of a molecule applies to the entire molecule. For example, 2H 2 O means that we have two water molecules. Since each water molecule has two hydrogen atoms, the total number of hydrogen atoms is four. It is always easiest to balance pure elements last. For example, if O 2 appears in a, you can change the coefficient to make 2O 2 or 3O 2 and change only the number of oxygen atoms. Changing CH 4 to 2CH 4 changes both the number of carbon atoms and the number of hydrogen atoms. Another good strategy is to start with one element, often carbon. Then check oxygen, and so on, checking and balancing one element at a time. However, every time you change a coefficient, you should make sure you have not unbalanced an element that you already balanced! The glucose (C 6 H 12 O 6 ) contained in biomass is used as a biofuel to produce ethanol (C 2 H 6 O) and carbon dioxide (CO 2 ). Write the balanced for this reaction. Asked: The coefficients needed to balance the Given: The unbalanced C 6 H 12 O 6 C 2 H 6 O + CO 2 Relationships: The same number of each type of atom must appear on each side. Solve: All atoms involved in the reaction are unbalanced. There are 6 C atoms in the reactants and 3 C atoms in the products. There are 12 H atoms in the reactants and 6 H atoms in the products. There are 6 O atoms in the reactants and 3 O atoms in the products. From this accounting, we notice that the number of atoms in the products is half the number of the reactants. By multiplying the products by a factor of 2 we obtain a balanced of the reaction. Answer: The balanced is C 6 H 12 O 6 2C 2 H 6 O + 2CO 2. A NATURAL APPROACH TO CHEMISTRY 301